1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine which is adapted to perform an air-fuel feedback control on the basis of air-fuel ratio signals generated by a pair of air-fuel ratio sensors mounted in an exhaust pipe at positions upstream and downstream of an exhaust gas purifying catalytic converter, respectively. More particularly, the invention is concerned with an air-fuel ratio control system which is capable of suppressing control errors brought about due to variation or deterioration of the operating (output) characteristic and/or response time of the air-fuel ratio sensor disposed upstream of the catalytic converter and which can prevent occurrence of delay and overshoot in the air-fuel ratio control. Further, the invention is concerned with an apparatus for detecting deterioration of an exhaust gas purifying catalytic converter on the basis of the output signals of the two air-fuel ratio sensors.
2. Description of the Related Art
In general, the amount of fuel injected into the internal combustion engine (hereinafter also referred to simply as the engine) is controlled through a feedback control loop on the basis of an output signal of an air-fuel ratio sensor such as O.sub.2 -sensor mounted in an exhaust pipe so that the air-fuel ratio (A/F) of the air-fuel mixture assumes a stoichiometrically optimal value (typically a value of about 14.7) for the prevailing operation state of the engine.
Oxygen concentration of the exhaust gas is usually low when the air-fuel ratio of the fuel mixture assumes a value which is smaller than the above-mentioned value of 14.7, indicating that the fuel mixture is rich, and if otherwise, the oxygen concentration is high. Accordingly, the air-fuel ratio sensor such as the O.sub.2 -sensor is so designed as to respond to the oxygen concentration of the exhaust gas by changing correspondingly the voltage level of the output signal of the sensor within a range from 0 to 1 volt around the level corresponding to the air-fuel ratio of 14.7. By way of example, when the air-fuel ratio indicates an excess of fuel (i.e., rich mixture with excess fuel), the voltage level of the output signal of the air-fuel ratio sensor (O.sub.2 -sensor) increases correspondingly in response to decrease in the oxygen concentration.
In the air-fuel ratio control system in which a single air-fuel ratio sensor is installed in the exhaust pipe at a position upstream from the catalytic converter, the accuracy of the air-fuel ratio control is affected remarkably by a change or deterioration of the output characteristic (operating or performance characteristic) of the air-fuel ratio sensor. More specifically, the operating characteristic of the air-fuel ratio sensor may differ from one to another sensor due to manufacturing tolerances (dispersion of the operating characteristic) and/or undergo deterioration in the course of time lapse. Under the circumstances, there has been proposed such an air-fuel ratio control system in which an additional air-fuel ratio sensor is provided at a position downstream from the catalytic converter for the purpose of correcting or improving the air-fuel ratio feedback control performed on the basis of the air-fuel ratio signal outputted from the air-fuel ratio sensor disposed upstream of the catalytic converter. In this regard, it is noted that the air-fuel ratio sensor disposed downstream of the catalytic converter detects the air-fuel ratio of the exhaust gas which has already undergone the catalytic reaction and which has a substantially uniform or constant oxygen concentration. Thus, the downstream sensor is less susceptible to deterioration or degradation of the operating (or output) characteristic thereof. For this reason, the dual-sensor type air-fuel ratio control system can perform the air-fuel ratio feedback control with significantly enhanced accuracy when compared with the single-sensor type control system, In other words, dispersion of the operating characteristics of the upstream air-fuel ratio sensor and the fuel injector as well as the time-dependent deterioration or degradation thereof can successfully be compensated for by utilizing the output signal of the downstream air-fuel ratio sensor. A typical one of such dual-sensor type air-fuel ratio control system is disclosed, for example, in U.S. Pat. No. 3,939,654.
For better understanding of the present invention, the background techniques will be elucidated below in some detail.
FIG. 24 is a block diagram showing a typical one of the conventional air-fuel ratio control system for an internal combustion engine in which a pair of air-fuel ratio sensors are disposed in an exhaust pipe upstream and downstream from a catalytic converter, respectively.
Referring to FIG. 24, an internal combustion engine (hereinafter referred to simply as the engine) 1 is provided with an intake pipe 2 for supplying an air-fuel mixture to the engine 1, an air cleaner 3 disposed at an inlet port of the intake pipe 2, an intake manifold 4 formed at a junction between the intake pipe 2 and the engine 1 and a fuel injector 5 mounted in the intake pipe 2 at a position upstream of a throttle valve 7. Further, mounted in the intake manifold 4 is a semiconductor-type pressure sensor 6 for detecting a pressure P within the manifold 4. This pressure P indicates an amount of the air-fuel mixture supplied to the engine 1 from the intake pipe 2 through the manifold 4. A throttle sensor 8 is provided in association with the throttle valve 7 for detecting the throttle opening degree .phi.. The engine 1 is further equipped with an exhaust pipe 9 for discharging an exhaust gas resulting from combustion of the air-fuel mixture within the engine. A ternary catalytic converter 10 is installed in the exhaust pipe 9 for eliminating three poisonous components HC, CO and NO.sub.x from the exhaust gas. A first air-fuel ratio sensor 11 (typically an O.sub.2 -sensor) is mounted in the exhaust pipe 9 at a position upstream of the catalytic converter 10 with a second air-fuel ratio sensor 12 (typically an O.sub.2 -sensor) being disposed downstream of the catalytic converter 10.
For the control of the engine operation, there is provided an electronic control unit (ECU) 20 having an input terminal to which an ignition coil 13 composed of a boosting transformer and an igniter 14 constituted by a power transistor for interrupting electric conduction through a primary winding of the ignition coil 13 is connected. An idle switch 15 which serves to detect the idling operation state of the engine in which the throttle valve 7 is fully closed is provided integrally with the throttle sensor 8, wherein the output signals D and .phi. of both the idle switch 15 and the throttle sensor 8 are supplied to the electronic control unit 20. Supplied additionally to the electronic control unit 20 are an output signal T of a thermistor-type water temperature sensor 16 which is employed for detecting the temperature T of the engine cooling water and an output signal P of the pressure sensor 6 mentioned above. An electric power for the electronic control unit 20 and other components is supplied from an onboard battery 17 via a key switch 18. An alarm lamp 19 driven upon detection of abnormality such as deterioration of the catalytic converter and the like (indicated by a signal E) is connected to an output terminal of the electronic control unit 20. Further, a vehicle speed sensor 40 for detecting a speed of a motor vehicle is installed in association with an axle. It goes without saying that the operation of the fuel injector 5 is controlled by the electronic control unit 20 in dependence on the running or operation state of the engine 1 while taking into account the air-fuel ratio detected by the sensors 11 and 12.
As will be apparent from the above description, the electronic control unit 20 receives as the engine operation state indicating parameters or quantities a variety of input signals including the throttle opening signal .phi. from the throttle sensor 8, the pressure signal P from the pressure sensor 6 indicating the pressure within the intake manifold 4, the coolant water temperature signal T from the water temperature sensor 16, the idle signal D from the idle switch 15, a vehicle speed signal S from the 40, a rotation or interrupt signal R generated upon every interruption of the electric conduction of the ignition coil 13 and serving as an interrupt signal, as described later on, and air-fuel ratio signals V1 and V2 supplied from the first and second air-fuel ratio sensors 11 and 12, respectively.
Operation of the electronic control unit 20 which may be realized on the basis of microcomputer techniques is activated in response to the power supply from the battery 17 which is enabled by closing the key switch 18, whereby the aforementioned engine operation state signals inclusive of the air-fuel ratio signals V1 and V2 are supplied to the electronic control unit 20, whereby a fuel injection control signal J is generated under the control through an air-fuel ratio feedback control loop. The abnormality signal E for lighting the alarm lamp 19 is generated upon detection of occurrence of abnormality (such as deterioration of the catalytic converter) in the engine operation.
FIG. 25 is a block diagram showing a functional configuration of the electronic control unit 20. As can be seen in the figure, the electronic control unit 20 includes a microcomputer 30, a first input interface 21 for receiving the rotation signal R to thereby generate an interrupt signal INT by shaping the waveform of the former, a second input interface 22 through which the air-fuel ratio signals V1 and V2, the pressure signal P, the cooling water temperature signal T and the throttle opening signal .phi. are fetched, a third input interface 23 for fetching the idle signal D and the vehicle speed signal S, an output interface 24 for outputting the abnormality signal E and the fuel injection control signal J, and a power supply circuit 25 connected to the battery 17 via the key switch 18, wherein the microcomputer 30 is connected to the input interfaces 21, 22 and 23, the output interface 24 and the power supply circuit 25.
The microcomputer 30 is comprised of a central processing unit (CPU) 31 for determining arithmetically or calculating an air-fuel ratio feedback control quantity (hereinafter also referred to as the air-fuel ratio control quantity) on the basis of the air-fuel ratio signals V1 and V2, a free-running counter 32 for detecting a rotation speed of the engine 1 on the basis of the rotation signal R and hence the interrupt signal INT fetched through the first input interface 21, a timer 33 for timing the various control operations, an analogue-to-digital or A/D converter 34 for converting the analogue signals inputted through the second input interface 22 (i.e., the air-fuel ratio signals V1 and V2, the pressure signal P, the cooling water temperature signal T and the throttle opening signal .phi.) into corresponding digital signals, an input port 35 for fetching the idle signal D and the vehicle speed signal S through the third interface 23, a random access memory or RAM 36 used as a work area for the CPU 31, a read-only memory or ROM 37 for storing programs in accordance with which the CPU 31 performs various operations, an output port 38 for outputting the fuel injection control signal J and the emergency or abnormality signal E, and a common bus 39 through which the various constituents 32 to 38 mentioned above are coupled to the CPU 31 for interactions therewith.
In response to the interrupt signal INT inputted through the first input interface 21, the CPU 31 reads a count value from the counter 32 and computes the rotation period or speed (rpm) of the engine 1 on the basis of a difference between the current count value and the preceding count value read upon input of the preceding interrupt signal INT. The engine rotation period thus determined is stored in the RAM 36.
The output interface 24 serves to amplify the control signal supplied from the output port 38 to thereby generate the fuel injection control signal as well as the abnormality signal E, as occasion requires.
FIG. 26 is a functional block diagram showing schematically functions involved in the air-fuel ratio feedback control performed by the microcomputer 30 of the conventional electronic control unit. In this figure reference numeral 41 denotes a first PI controller for effecting a PI (proportional-pulse-integral) action or control for the air-fuel ratio signal V1 supplied from the first or upstream air-fuel ratio sensor 11. Further, reference numeral 42 denotes a second PI controller for effecting a PI action or control for the air-fuel ratio signal V2 supplied from the second or downstream air-fuel ratio sensor 12.
Each of the PI controllers 41 and 42 constitutes an arithmetic means for generating air-fuel ratio control quantities C1 and C2 on the basis of the air-fuel signals V1 and V2, respectively, wherein the second air-fuel ratio control quantity C2 is used as an air-fuel ratio correcting quantity for correcting the first air-fuel ratio control quantity C1. On the other hand, the first air-fuel ratio control quantity C1 serves as an air-fuel ratio correcting quantity for correcting the air-fuel ratio V1 through a feedback control described hereinafter. The fuel injection control signal J ultimately generated on the basis of the control quantity C1 is supplied to the fuel injector 5 through the feedback control which controls the fuel injection so that the second air-fuel ratio signal V2 coincides with a desired or target value (second target value) VR2 set for the second air-fuel ratio sensor 12.
More specifically, for the first and the second air-fuel ratio signals V1 and the V2, there are previously determined and set first and second target values VR1 and VR2 for the air-fuel ratio control, respectively, for each of which a voltage value substantially corresponding to an stoichiometrically optimal air-fuel ratio 14.7 is set. However, the voltage value for the second target value VR2 may be set slightly higher than the first target value VR1 so as to indicate an air-fuel ratio smaller than 14.7 (i.e., excess in fuel).
A basic fuel amount signal FR shown in FIG. 26 is arithmetically determined on the basis of the pressure signal P (FIGS. 24, 25) which represents equivalently the amount of intake air. Further, in FIG. 26, reference symbols CF designates a fuel injection correcting quantity determined on the basis of the water temperature signal T and the throttle opening signal .phi. in the acceleration/deceleration mode of the engine, KF designates an injection time correcting coefficient for the fuel injector 5 for correcting the fuel injection time or period on the basis of a desired amount of fuel injection, and Q designates a dead time correcting quantity for correcting the driving time of the fuel injector 5 by taking into account the dead time.
In operation, a difference .DELTA.V2 between the second target value VR2 and the second air-fuel ratio signal V2 is determined by a subtractor 43 and applied to the input of the second PI controller 42. The second air-fuel ratio control quantity C2 outputted from the controller 42 after undergoing the PI action is added to the first target value VR1 by an adder 44, whereby a correction target quantity VT1 is generated. Further, a difference .DELTA.V1 between the correction target quantity VT1 and the first air-fuel ratio signal V1 is determined by a subtractor 45 and inputted to the first PI controller 41. The adder 44 thus constitutes a correcting means for correcting the first air-fuel ratio control quantity C1 determined by the first PI controller 41 by utilizing the second air-fuel ratio control quantity C2.
The first air-fuel ratio control quantity C1. outputted from the first PI controller 41 is multiplied by the basic fuel amount FR in a multiplier 46, whereby a target fuel amount signal F1 is generated to be subsequently multiplied by the fuel correction quantity CF in a multiplier 47, as a result of which a signal representing a corrected fuel amount signal F is generated. The corrected fuel amount signal F is multiplied by the fuel injection time KF by a multiplier 48, whereby a signal indicating a driving time G for the fuel injector 5 is generated. The dead time correction quantity Q is added to the driving time signal G by an adder 49, which results in generation of the fuel injection control signal J to be ultimately obtained. The multipliers 46 to 48 and the adder 49 constitute a control quantity conversion means for converting the first air-fuel ratio control quantity C1 into the fuel injection control signal J.
Next, referring to FIG. 24 to 26 along with a waveform diagram shown in FIG. 27, operation of the air-fuel ratio control system known heretofore will be described in detail.
The subtractor 43 determines a deviation or difference .DELTA.V2(=VR2-V2) by comparing the second air-fuel ratio signal V2 derived from the output of the second air-fuel ratio sensor 12 installed downstream of the catalytic converter 10 with the second target value VR2, while the second PI controller 42 computes or arithmetically determines the second air-fuel ratio control quantity C2 through the PI action performed on the difference .DELTA.V2.
On the other hand, the adder 44 adds the second air-fuel ratio control quantity C2 (i.e., a correcting quantity) to the first target value VR1, to thereby generate the correction target quantity VT1(=VR1+C2) for the output V1 of the first air-fuel ratio sensor 11. Further, the subtractor 45 determines a deviation or difference .DELTA.V1(=VT1-V1) by comparing the correction target quantity VT1 with the first air-fuel ratio signal V1 of the first air-fuel ratio sensor 11 disposed upstream of the catalytic converter 10, while the first PI controller 41 determines the first air-fuel ratio control quantity C1 for the feedback control by performing PI action on the difference .DELTA.V1.
In this manner, the first air-fuel ratio control quantity C1 which is based on the first air-fuel ratio signal V1 is derived through the correction based on the second air-fuel ratio control quantity C2, whereby the ultimate air-fuel ratio control quantity (fuel injection control quantity) J is obtained through the subsequent processings performed by the control quantity conversion means (46-49), as mentioned previously.
As is shown in FIG. 27, the signal representing the first air-fuel ratio control quantity C1 has a waveform approximately similar to that of the first air-fuel ratio signal V1 in respect to the period and the number of times at which the former intersects the correction target quantity VT1. On the other hand, the second air-fuel ratio signal V2 changes gently or flatly around the target value VR2.
In the meanwhile, on the basis of the pressure signal P derived from the output of the pressure sensor 6, the intake air amount is detected, on the basis of which the basic fuel amount FR is determined. The first air-fuel ratio control quantity C1 is multiplied by the basic fuel amount FR by means of the multiplier 46 to thereby determine the target fuel amount F1, as mentioned previously.
Subsequently, on the basis of the temperature T, a correcting quantity indicative of the warmed-up state of the engine 1 is fetched, which is then followed by detection of the acceleration/deceleration state of the engine 1 on the basis of the throttle opening degree .phi. derived from the output of the throttle sensor 8, whereby the fuel injection correcting quantity CF is determined by taking into account a correcting quantity which reflects the engine acceleration/deceleration state. The target fuel amount F1 is then multiplied by the fuel injection correcting quantity CF, to thereby determine the corrected fuel injection amount F which represents the fuel injection amount to be ultimately determined.
The multiplier 48 multiplies the corrected fuel injection amount F by the fuel injection time correcting coefficient KF to thereby determine the driving time G of the fuel injector 5, while the adder 49 adds the dead time correction quantity Q to the driving time G to determine the ultimate fuel injection control signal J for controlling the fuel injector 5.
As is apparent from the above description, the air-fuel ratio feedback control is performed such that the air-fuel ratio signal V2 outputted from the second air-fuel ratio sensor 12 disposed downstream of the catalytic converter 10 can take the second target value VR2 by correcting the first target value VR1 for the first air-fuel ratio sensor 11 with the correcting quantity VT1 generated on the basis of the second air-fuel ratio signal V2 outputted from the second air-fuel ratio sensor 12. By way of example, when the second air-fuel ratio signal V2 of the second air-fuel ratio sensor 12 disposed downstream of the catalytic converter 10 indicates that the air-fuel ratio is smaller than 14.7 and hence the air-fuel mixture is rich, duration of the fuel injection control signal J is so set as to become shorter, whereby the fuel injection is controlled so that the air-fuel mixture becomes lean.
At this juncture, it is however noted that the first air-fuel ratio sensor 11 as well as the second air-fuel ratio sensor 12 can not evade dispersion in the operating or output characteristic, as pointed out hereinbefore. In other words, the operating characteristics of these sensors may differ from one to another due to intrinsic error or tolerance in the manufacturing. Besides, the operating characteristic of the first air-fuel ratio sensor 11 disposed upstream of the catalytic converter 10 undergoes change or degradation as a function of time due to exposure to the poisonous components contained in the exhaust gas. Next, problems which may be brought about by the dispersion or deviation of the operating characteristic of the sensor as well as deterioration of the sensor element will be elucidated below.
FIG. 28 is a waveform diagram which illustrates in what manner the air-fuel ratio sensors of different characteristics respond in case the air-fuel ratio (A/F) is forcibly changed. More specifically, a waveform Va of FIG. 28 represents a response characteristic of a standard air-fuel ratio sensor having a standard response characteristic, a waveform Vb represents a response characteristic of an air-fuel ratio sensor exhibiting an operating characteristic deviated form that of the standard and a waveform Vc represents a response characteristic of an air-fuel ratio sensor undergone deterioration or degradation.
Referring to FIG. 28, the air-fuel ratio (A/F) was changed from a value indicating a lean mixture to a value corresponding to a rich mixture around the target (or stoichiometrically optimal) value of 14.7. In that case, the air-fuel ratio sensor having the standard characteristic Va responds with a delay time of about 100 milliseconds as can be seen from the curve Va. On the other hand, the air-fuel ratio sensor having the response characteristic Vb deviated from the standard responds with a delay of about 200 milliseconds, as indicated by the curve Vb. Finally, the response of the air-fuel ratio sensor of the deteriorated the characteristic takes as long a time as about 1.2 second at maximum, as indicated by a curve Vc.
FIG. 29 is a waveform diagram for illustrating air-fuel ratio feedback control operations effected when the air-fuel ratio sensors having the response characteristics Va to Vc shown in FIG. 28 are, respectively, used as the first air-fuel ratio sensor 11.
Referring to FIG. 29, it is assumed that the first target value VR1 for the first air-fuel ratio signal V1 has a value .alpha. with the correction target quantity VT1 having a value .beta. and that the value of the first target value VR1 is to be corrected from the value .alpha. to .beta. by utilizing the air-fuel ratio control quantity C2 which is based on the second air-fuel ratio signal V2. More specifically, it is assumed that the second air-fuel ratio signal V2 indicates that the air-fuel mixture is richer than that corresponding to the second target value VR2 (the second air-fuel ratio signal V2 is of a higher voltage than the signal representing the value VR2), wherein the second air-fuel ratio control quantity C2 of a negative (minus) value is added to the first target value VR1 to thereby lower the first target value VR1, so that the air-fuel ratio as detected indicates a lean air-fuel mixture (this ratio will hereinafter be referred to as the lean air-fuel ratio).
In FIG. 29, reference symbols T.alpha. and T.beta. represent the times taken for the comparisons of the first air-fuel ratio signal V1 with the correction target values .alpha. and .beta., respectively, to determine richness of the air-fuel mixture (the time involved in this decision will hereinafter be referred to as the richness decision time).
The first air-fuel ratio control quantity C1 which is based on the output of the first air-fuel ratio sensor 11 having the standard characteristic (Va) mentioned above changes in such a manner as indicated by a broken line curve, when the richness decision takes a time T.beta. which is longer than T.alpha. for deciding that the air-fuel mixture becomes lean.
Similarly, it is assumed that when the first air-fuel ratio sensor 11 having the output characteristic Vb the characteristic deviated from the standard) or the first air-fuel ratio sensor 11 exhibiting the characteristic Vc (due to deterioration of the sensor element) is used and that the richness decision time is changed from T.alpha. to T.beta.. In this case, it is however noted that because of differences in the output characteristics of these air-fuel ratio sensors as indicated by a broken curve Vb and a dot-broken curve respectively, the response times of these sensors differ from each other and from that of the standard sensor (Va), as a result of which difference of the change (T.beta.-T.alpha.) in the rich decision time differs between the case where the first air-fuel ratio sensor 11 having the standard characteristic (Va) is used and the case where the first air-fuel ratio sensor 11 having the deviated characteristic (Vb) or the degraded characteristic (Vc) is employed.
As will be understood from the foregoing, when the output characteristic of the first air-fuel ratio sensor 11 differs from that of the sensor having the standard characteristic, the difference between the richness decision times T.alpha. and T.alpha. is deviated from that of the standard sensor correspondingly in dependence on magnitude of the difference (.beta.-.alpha.) of the first target value VR1 due to correction thereof by the second air-fuel ratio control quantity C2. This means that the first air-fuel ratio control quantity C1 determined ultimately differs correspondingly from that obtained when the air-fuel ratio sensor of the standard characteristic is used as the first air-fuel ratio sensor 11.
As is apparent from the above description, when the output characteristic and the response time of the first air-fuel ratio sensor 11 are deviated from the standard or deteriorated as a function of time lapse, the first air-fuel ratio control quantity C1 obtained on the basis of the first target value VR1 corrected by the second air-fuel ratio control quantity C2 becomes different from that of the air-fuel ratio sensor of the standard characteristic ultimately making it impossible to correct the air-fuel ratio in a satisfactory manner. More specifically, when the first air-fuel ratio sensor 11 having the standard output characteristic (Va) is used, the response time is comparatively short. As a consequence, the richness decision time based on the second air-fuel ratio control quantity C2 and the first target value VR1 is short. This means hat the second air-fuel ratio control quantity C2 for the first target value VR1 should be set at a greater value for the correct air-fuel ratio control. On the other hand, when the first air-fuel ratio sensor 11 having the degraded output characteristic (Vc) is used, the rich decision time becomes longer than the case where the first air-fuel ratio sensor 11 having the standard output characteristic (Va) is used, making it impossible to realize the appropriate air-fuel ratio control with the control quantity C2 set for the standard characteristic. Same holds true also for the first air-fuel ratio sensor having the deviated characteristic (Vb). Moreover, since the first air-fuel ratio sensor 11 disposed upstream of the catalytic converter 10 is easily susceptible to deterioration, the unwanted changes will take place in the first air-fuel ratio control quantity C1, which can not be neglected.
As will now be appreciated from the foregoing description, the air-fuel ratio control system for the engine known heretofore suffers from a problem that magnitude of the first air-fuel ratio control quantity C1 which is to be ultimately obtained on the basis of the second air-fuel ratio control quantity C2 derived from the second air-fuel ratio signal V2 and the first target value VR1 for the first air-fuel ratio signal V1 becomes different in dependence on deviation or degradation of the output characteristic and the response time of the sensor employed as the first air-fuel ratio sensor 11. Thus, it is impossible to correct the air-fuel ratio control quantity in a satisfactory manner unless the operating or output characteristic of the first air-fuel ratio sensor 11 actually employed is taken into account and compensated for properly, as occasion requires.
Another problem of the air-fuel ratio control system known heretofore can be seen in conjunction with the second air-fuel ratio sensor 12 as well. Certainly, the second air-fuel ratio sensor 12 is low in the response speed when compared with that of the first air-fuel ratio sensor. However, since the temperature of the exhaust gas is low at the side downstream of the catalytic converter, the second air-fuel ratio sensor 12 is less susceptible to the influence of the heat. Besides, because the exhaust gas is purified by the catalytic converter, the second air-fuel ratio sensor 12 is scarcely exposed to the poisonous components contained in the exhaust gas. Furthermore, the exhaust gas is sufficiently mixed so that the oxygen concentration of the exhaust gas is held substantially equilibrium, as can be seen from the graph shown atop in FIG. 27. Thus, the second air-fuel ratio sensor can enjoy a stable output characteristic which is advantageous for compensating for deviations in the output characteristic of the first air-fuel ratio sensor.
However, since the air-fuel ratio feedback control is performed on the basis of the integral operation of the output of the second air-fuel ratio sensor, there arise problems such as a time lag in the air-fuel ratio control, overshoot correction of the air-fuel ratio and hence degradation in the cost-performance, exhaust gas emission characteristic and the drivability of the motor vehicle. Additionally, in the dual-sensor type air-fuel ratio control apparatus known heretofore, the amplitude of the first air-fuel ratio signal is not taken into consideration in the correction of the air-fuel ratio, incurring an improper air-fuel ratio feedback control, to another disadvantage
Still another problem of the air-fuel ratio control system known heretofore is seen in that the function or performance of the catalytic converter 10 may significantly be degraded when it is exposed to unburned fuel upon occurrence of misfire or for other reason. In that case, the motor vehicle continues to run with the exhaust gas remaining scarcely purified by the catalytic converter, which in turn brings about change in the output characteristic of the second air-fuel ratio sensor 12 disposed downstream under the influence of the unburned fuel components such as HC, CO, H.sub.2 and others. By way of example, the rate of change of the air-fuel ratio signal outputted from the downstream air-fuel ratio sensor may increase, incurring eventually significant degradation in the emission characteristic (i.e., exhaust gas purification performance).
For the above reason, it is important to detect functional degradation of the catalytic converter 10 and alarm the driver of this fact as soon as possible when the malfunction of the catalytic converter 10 is detected. In this conjunction, there is disclosed an apparatus for detecting degradation of a catalytic converter in Japanese Unexamined Patent Application No. 264312/1991 (JP-A-H3-264312) assigned to the assignee of the present application. In the case of this known apparatus, the air-fuel ratio signal of the air-fuel ratio sensor disposed downstream of the catalytic converter is compared with a reference level, wherein decision as to degradation of the catalytic converter is made on the basis of an integral value .delta. of deviation of the air-fuel ratio signal of the downstream air-fuel ratio sensor, the number of times the air-fuel ratio signal intersects the reference level, the period .tau. or a time interval ratio .gamma.=.tau..sub.1 /.tau..sub.2 (where .tau..sub.1 represents the period of the air-fuel ratio signal of the upstream sensor 11 with .tau..sub.2 representing that of the downstream sensor 12 and others).
The catalyst degradation detecting technique known heretofore will be elucidated in more concrete by again referring to FIG. 24. The alarm lamp 19 for indicating occurrence of abnormality is driven upon detect, ion of degradation in the performance of the catalytic converter 10. Further, the electronic control unit 20 incorporates a catalyst degradation decision means which generates the abnormality signal E indicative of degradation of the catalytic converter 10 upon detection of degradation thereof, for thereby lighting the alarm lamp 19.
Operation of the catalyst degradation decision means which can be implemented softwarewise will now be considered. FIG. 30 shows waveforms of the first and second air-fuel ratio signal V1 and V2 when the catalytic converter 10 operates normally, while FIG. 31 shows the same when the catalytic converter suffers abnormality. As can be seen from these figures, amplitude of the first air-fuel ratio signal V1 changes relative to the correction target quantity VT1 periodically at a proper time interval regardless of whether the catalytic converter 10 is normal or abnormal owing to the air-fuel ratio feedback control described hereinbefore.
On the other hand, the second air-fuel ratio signal V2 which remains substantially uniform owing to the purifying action of the catalytic converter 10 as mentioned previously and shown in FIG. 30 changes significantly in following the first air-fuel ratio signal V1 when the exhaust gas purifying capability of the catalytic converter 10 is lowered as can clearly be seen by comparing FIG. 30 with FIG. 31. Thus, it is possible to detect abnormality or degradation of the catalytic converter 10 on the basis of the integral value .delta. (FIG. 31), the period .tau. of the second air-fuel ratio signal V2 or the number n of times the signal V2 intersects the second target value VR2 (this number will hereinafter be referred to also as the frequency).
Further, when the time interval ratio .gamma.(=.tau..sub.1 /.tau..sub.2) is employed as the degradation decision parameter, as mentioned previously, degradation of the catalytic converter 10 can be determined when the time interval ratio .gamma. exceeds a reference value .gamma.o (about 50% of the maximum value).
In this regard, however, it should be recalled that the first air-fuel ratio sensor 11 disposed upstream of the catalytic converter 10 is susceptible to deterioration for the reasons described hereinbefore. Besides, dispersion in the operating characteristic of the sensors used as the first air-fuel ratio sensor 11 can not be avoided. As a consequence, there may arise the possibility that the second air-fuel ratio signal V2 is shifted to such extent that it can no more intersect the second target value VR2 as indicated by a dotted curve in FIG. 30, which means that neither the integral value .delta. nor the frequency n nor the period .tau. can no more be determined. In other words, when the output characteristic of the first air-fuel ratio signal V1 of the first air-fuel ratio sensor 11 changes, it may become impossible to detect degradation of the catalytic converter 10 on the basis of the output signal of the second air-fuel ratio sensor 12. Besides, since the air-fuel ratio signals V1 and V2 vary in dependence on changes in the operation state of the engine, involving changes in the values of .delta., .tau. and .gamma. as determined, which may lead to erroneous decision of degradation of the catalytic converter 10.
It can now be understood that the catalyst degradation detecting apparatus known heretofore suffers a drawback that the detection reliability is poor.